1. Field of the Invention
This invention relates generally to image display apparatus and is concerned more particularly with an imaging system for producing three different color component images of the same subject matter and combining the images to display a single multi-color image of said subject matter.
2. Discussion of the Prior Art
A cathode ray type of image display tube generally includes a tubular envelope having therein an electron gun disposed for directing an electron beam in scanning fashion over a portion of an output phosphor screen to produce a visible light image. The phosphor screen may be provided with a suitable phosphor material for emitting a desired color light when penetrated by electrons from the scanning beam. Thus, a plurality of cathode ray tubes may have respective phosphor screens comprised of different phosphor materials for producing respective different color component images of the same subject matter, and be disposed for projecting the different color images into superposition with one another on a projection screen. As a result, there will be displayed on the projection screen a single multi-color image having discrete areas colored in accordance with the relative intensities of the different colors in corresponding discrete areas of the respective component images produced by each of the cathode ray tubes.
However, it has been found difficult to provide a plurality of cathode ray tubes with means for producing identical images of the same subject matter, and to maintain the tubes in precise optical alignment so as to obtain color registration in discrete areas of the multi-color image. Consequently, there has been developed in the prior art various types of cathode ray tubes having phosphor screens provided with different phosphor materials for producing a multi-color image. A cathode ray tube of the shadow mask type, for example, has a phosphor screen provided with intermingled arrays of phosphor dots, the dots of each array comprising a different phosphor material for emitting a respective color light when penetrated by electrons from an associated beam passing through aligned apertures in a shadow mask. A cathode ray tube of the beam-indexing type, for another example, has a phosphor screen provided with intermingled arrays of phosphor stripes, the stripes of each array comprising a different phosphor material for localized emission of a respective color light when penetrated by electrons from a suitably indexed electron beam. A cathode ray tube of the voltage penetration type, for a third example, has a phosphor screen provided with superimposed layers of different phosphor material, the material of each layer being activated for localized emission of a respective color light when penetrated by electrons of an associated energy level in a scanning electron beam.
Thus, cathode ray tubes of the described types have phosphor screens requiring complex deposition techniques or special phosphor materials during fabrication, which increases the cost of producing these prior art tubes.